A major portion of the worldwide petrochemical industry is concerned with the production of light olefin materials and their subsequent use in the production of numerous important chemical products via polymerization, oligomerization, alkylation and the like well-known chemical reactions. Light olefins include ethylene, propylene and mixtures thereof. These light olefins are essential building blocks for the modern petrochemical and chemical industries. The major source for these materials in present day refining is the steam cracking of petroleum feeds. For various reasons including geographical, economic, political and diminished supply considerations, the art has long sought a source other than petroleum for the massive quantities of raw materials that are needed to supply the demand for these light olefin materials. In other words, the holy grail of the R & D personnel assigned to work in this area is to find a way to effectively and selectively use alternative feedstocks for this light olefin production application thereby lessening dependence of the petrochemical industry on petroleum feedstocks. A great deal of the prior art's attention has been focused on the possibility of using hydrocarbon oxygenates and more specifically methanol as a prime source of the necessary alternative feedstock. Oxygenates are particularly attractive because they can be produced from such widely available materials as coal, natural gas, recycled plastics, various carbon waste streams from industry and various products and by-products from the agricultural industry. The art of making methanol and other oxygenates from these types of raw materials is well established and typically involves the use of one or more of the following procedures: (1) manufacture of synthesis gas by any of the known techniques typically using a nickel or cobalt catalyst followed by the well-known methanol synthesis step using relatively high pressure with a copper-based catalyst; (2) selective fermentation of various organic agricultural products and by-products in order to produce oxygenates; or (3) various combinations of these techniques.
Given the established and well-known technologies for producing oxygenates from alternative non-petroleum raw materials, the art has focused on different procedures for catalytically converting oxygenates such as methanol into the desired light olefin products. These light olefin products that are produced from non-petroleum based raw materials must of course be available in quantities and purities such that they are interchangeable in downstream processing with the materials that are presently produced using petroleum sources. Although many oxygenates have been discussed in the prior art, the principal focus of the two major routes to produce these desired light olefins has been on methanol conversion technology primarily because of the availability of commercially proven methanol synthesis technology. A review of the prior art has revealed essentially two major techniques that are discussed for conversion of methanol to light olefins. The first of these MTO processes is based on early German and American work with a catalytic conversion zone containing a zeolitic type of catalyst system. Representative of the early German work is U.S. Pat. No. 4,387,263 which was filed in May of 1982 in the U.S. without a claim for German priority. This '263 patent reports on a series of experiments with methanol conversion techniques using a ZSM-5-type of catalyst system wherein the problem of DME recycle is a major focus of the technology disclosed. Although good yields of ethylene and propylene were reported in this '263 patent, they unfortunately were accompanied by substantial formation of higher aliphatic and aromatic hydrocarbons which the patentees speculated might be useful as an engine fuel and specifically as a gasoline-type of material. In order to limit the amount of this heavier material that is produced, the patentees of the '263 patent propose to limit conversion to less than 80% of the methanol charged to the MTO conversion step. This operation at lower conversion levels necessitated a critical assessment of means for recovering and recycling not only unreacted methanol but also substantial amounts of a DME intermediate product. The focus then of the '263 patent invention was therefore on a DME and methanol scrubbing step utilizing a water solvent in order to efficiently and effectively recapture the light olefin value of the unreacted methanol and of the intermediate reactant DME.
This early MTO work with a zeolitic catalyst system was then followed up by the Mobil Oil Company who also investigated the use of a zeolitic catalyst system like ZSM-5 for purposes of making light olefins. U.S. 4,587,373 is representative of Mobil's early work and it acknowledged and distinguished the German contribution to this zeolitic catalyst based MTO route to light olefins. The inventor of the '373 patent made two significant contributions to this zeolitic MTO route the first of which involved recognition that a commercial plant would have to operate at pressure substantially above the preferred range that the German workers in this field had suggested in order to make the commercial equipment of reasonable size when commercial mass flow rates are desired. The '373 patent recognized that as you move to higher pressure for the zeolitic MTO route in order to control the size of the equipment needed for commercial plant there is a substantial additional loss of DME that was not considered in the German work. This additional loss is caused by dissolution of substantial quantities of DME in the heavy hydrocarbon oil by-product recovered from the liquid hydrocarbon stream withdrawn from the primary separator. The other significant contribution of the '373 patent is manifest from inspection of the flow scheme presented in FIG. 2 which prominently features a portion of the methanol feed being diverted to the DME absorption zone in order to take advantage of the fact that there exist a high affinity between methanol and DME thereby downsizing the size of the scrubbing zone required relative to the scrubbing zone utilizing plain water that was suggested by the earlier German work.
Primarily because of an inability of this zeolitic MTO route to control the amounts of undesired C4+ hydrocarbon products produced by the ZSM-5 type of catalyst system, the art soon developed a second MTO conversion technology based on the use of a non-zeolitic molecular sieve catalytic material. This branch of the MTO art is perhaps best illustrated by reference to UOP's extensive work in this area as reported in numerous patents of which U.S. Pat. No. 5,095,163, U.S. Pat. No. 5,126,308 and U.S. Pat. No. 5,191,141 are representative. This second approach to MTO conversion technology was primarily based on using a catalyst system comprising a non-zeolitic molecular sieve, generally a metal aluminophosphate (ELAPO) and more specifically a silicoaluminophosphate molecular sieve (SAPO), with a strong preference for a SAPO species that is known as SAPO-34. This SAPO-34 material was found to have a very high selectivity for light olefins with a methanol feedstock and consequently very low selectivity for the undesired corresponding light paraffins and the heavier materials. This ELAPO catalyzed MTO approach is known to have at least the following advantages relative to the zeolitic catalyst route to light olefins: (1) greater yields of light olefins at equal quantities of methanol converted; (2) capability of direct recovery of polymer grade ethylene and propylene without the necessity of the use of extraordinary physical separation steps to separate ethylene and propylene from their corresponding paraffin analogs; (3) sharply limited production of by-products such as stabilized gasoline; (4) flexibility to adjust the product ethylene-to-propylene weight ratios over the range of 1.5:1 to 0.75:1 by minimal adjustment of the MTO conversion conditions; and (5) significantly less coke make in the MTO conversion zone relative to that experienced with the zeolitic catalyst system.
For various reasons well articulated in UOP's patents, U.S. Pat. No. 6,403,854, U.S. Pat. No. 6,166,282 and U.S. Pat. No. 5,744,680 (all of the teaching of which are hereby specifically incorporated by reference) the consensus of the practitioners in this OTO or MTO art points to the use of a fluidized reaction zone along with our associated fluidized regeneration zone as the preferred commercial solution to the problem of effectively and efficiently using an ELAPO or SAPO-type of catalyst system in this type of service. As is well-understood by those of skill in the fluidization art, the use of this technology gives rise to a substantial problem of solid-vapor separation in order to efficiently separate the particles of the fluidized catalyst from the vapor products of the OTO or MTO reaction as well as from any unreacted oxygenate materials exiting the OTO or MTO conversion zone. Standard industry practice for accomplishing this difficult separation step involves its use of one or more vapor-solid cyclonic separating means which are well illustrated in the sole drawing of U.S. Pat. No. 6,166,282 where a series of three cyclonic separation means are used to separate spent OTO or MTO catalyst from the product effluent stream. As is clear from the teachings of these three UOP patents as well as the teachings of U.S. Pat. No. 6,121,504 and US-A-2003/0088136 these still remain a very substantial problem of OTO or MTO catalyst contamination of the product effluent stream withdrawn from the fluidized conversion zone.
Despite the promising developments associated with the ELAPO or SAPO catalyzed routes to light olefins there are still substantial hurdles to overcome before an economically attractive OTO or MTO process can be fully realized. One very substantial economic problem is associated with the amount of fresh catalyst that must be added to the OTO or fluidized conversion zone in order to maintain the catalyst inventory in the OTO conversion system at design levels when the product effluent stream from the OTO conversion zone contains substantial amounts of contaminating catalyst particles which in the processes of the prior art discussed above are not recovered and recycled to the OTO conversion zone. This problem of effluent contamination by catalyst particles is made more significant in the non-zeolitic catalyzed route to the desired light olefins because of the relatively expensive nature of the ELAPO or SAPO molecular sieves used therein compared to the corresponding zeolitic molecular sieve, ZSM-5, which has been used and exemplified in many of the prior art OTO conversion processes. Current economic conditions are such that the cost of an equivalent amount of an ELAPO-containing catalyst system is expected to differ from the cost of the prior art zeolitic system by a factor of about 5 to 40 even considering the expected substantial savings in costs that will be associated with the large scale production of ELAPO molecular sieve for this particular application. The problem addressed by the present invention is then to provide a method for recovery and recycle of these effluent-contaminating catalyst particles that are present in the product effluent stream withdrawn from an OTO conversion zone that utilizes a fluidized transport bed system in combination with a relatively expensive ELAPO molecular sieve-containing catalyst system. In other words, the problem addressed by the present invention is to staunch the loss of catalyst particles from a fluidized OTO conversion zone operated with a relatively expensive catalyst system containing an ELAPO molecular sieve in order to decrease the consumption of the relatively expensive catalyst system and thereby improve the economics of the resulting OTO or MTO conversion process.
The solution envisioned and provided by the present invention to this catalyst loss problem involves the use of a wet scrubbing step designed to recover substantially all of the product effluent contaminating catalyst particles and to provide a slurry of these catalyst particles in a scrubbing liquid such as water with subsequent recycle of at least a portion of the catalyst particles contained in the resulting slurry to the OTO conversion zone or to the associated deactivated OTO catalyst regeneration zone thereby recapturing the catalytic activity of these contaminating catalyst particles and diminishing the need for fresh catalyst that must be added to the system in order to make-up for this source of catalyst losses. The key developments that enable the instant invention are our findings that the catalytic activity of these effluent-contaminating catalyst particles will survive not only the hydrothermal shock associated with the introduction of these relatively hot particles contained in this product effluent stream into a relatively cool wet scrubbing zone which captures these particles by immersion in a scrubbing liquid which is typically aqueous but also the direct return of these particles, after an optional concentration step, to either the relatively hot OTO conversion zone or to the relatively hot spent OTO catalyst regeneration zone without any additional treatment. The fact that the catalytic activity of these recaptured catalyst particles will survive the hydrothermal shocks associated with introduction into the wet scrubbing zone as well as the thermal shocks associated with return to either the OTO conversion zone or the associated catalyst regeneration zone are quite surprising in view of the substantial prejudice in the prior art to exposure of ELAPO-containing molecular sieve to immersion in a liquid such as water. For example, U.S. Pat. No. 5,744,680 discloses the use of a wet scrubbing step on the cooled effluent stream from an OTO conversion zone in order to remove ELAPO molecular sieve-containing catalyst particles from this effluent stream and prepare this effluent stream for downstream compression. However, the '680 patent merely teaches at column 8, lines 38 to 48 that the catalyst-containing bottom stream from the wet scrubbing step is withdrawn via line 28 from scrubbing zone 104 for further treatment which is unspecified. There is no teaching indicating that the activity value of these catalyst particles recovered from this wet scrubbing step can be recaptured for use in the OTO conversion zone. With reference to the drawing of the '680 patent the wet scrubbing zone is shown as zone 104 with injection of a scrubbing fluid comprising H2O via line 24 and recovery of a bottom slurry material containing a mixture of catalyst finds and water via line 2. Further evidence of the art's failure to recognize that the catalytic activity of these effluent-contaminating catalyst particles can be recaptured and reused is shown in U.S. Pat. No. 6,121,504 wherein a wet scrubbing is used for the purposes of quenching the effluent stream from the OTO conversion zone to produce a bottom stream which is cooled and recirculated to the wet scrubbing stream with a drag stream withdrawn from the recirculating scrubbing fluid and charged to a stripping zone for purposes of heat recovery. With reference to the drawing of the '504 patent the scrubbing quench zone is zone 13 and the catalyst-containing bottom stream is represented by line 15 with the drag stream being taken from line 17 after pump 16 is used to change the pressure of this circulating scrubbing liquid. A careful reading of the teachings of the '504 patent has failed to reveal any discussion of the presence or disposition of catalyst particles that are inherently entrained in the bottom stream from zone 13 when the OTO conversion zone 10 is operated in a fluidized mode. The sole example of the '504 patent exemplifies a fluidized OTO conversion operation without any discussion of the fate of the catalyst particles that are inherently entrained in the product effluent stream recovered therefrom. In a subsequent published application US2003/0088136A1 emanating from the same ExxonMobil Chemical Company responsible for the '504 patent, the teachings of the '504 patent are characterized and distinguished in paragraph 0007 by pointing out that the '504 patent “provides no guidance on how to manage the catalyst particles that exit the reactor entrained with the gaseous effluent stream.” Additional evidence that the prior art failed to recognize that the catalyst particles recovered from the product effluent stream of OTO process could be recovered by wet scrubbing and recycled to the OTO conversion zone without loss of their catalytic activity is shown in U.S. Pat. No. 6,403,854 wherein FIG. 2 exemplifies a two-stage quench arrangement for the hot effluent stream recovered from the OTO conversion zone and wherein in fact the first stage is a wet scrubbing zone employed for the purposes of neutralizing acidic materials that are by-product of the OTO conversion reaction and of removing catalyst fines entrained in the product effluent stream. The teachings of the '854 patent on this subject are concisely set forth in the first full paragraph of column 10 wherein the operation of wet scrubbing zone 42 (called therein first stage quench tower) is described. The scrubbing liquid is introduced into column 42 via line 24 and the resulting bottom stream is divided and a portion is used as a pump-around stream via lines 23 and 24 to act as the scrubbing liquid utilized in zone 42 with a drag stream withdrawn via lines 23 and 25 and this drag stream comprising the majority of the impurities and catalyst fines is concentrated into a small stream which is taught as comprising about 5 to about 10 wt-% of the total recovered water. Careful reading of the '854 patent has failed to reveal any hint or suggestion of possible recovery and reuse of the catalyst particles that are contained in the drag stream from the first stage of the two-stage quench zone exemplified therein. Additional convincing evidence of the prejudice in the prior art against recovery and reuse of the effluent contaminating catalyst particles is evident from a reading of the previously mentioned application publication from ExxonMobil Chemical Company, US2003/0088136A1, which was relatively recently published (i.e. May 8, 2003) and fully discloses the problem of product effluent contamination with catalyst particles in a section starting with paragraph 0037 and running through paragraph 0047 wherein the contamination of this product effluent stream is described and quantified. The use of a wet scrubbing zone in order to remove these contaminating particles is described in the '136 application on page 7 beginning at paragraph 0064 running through paragraph 0070. In the last sentence of 0070 applicants teach that the quench device serves two purposes in a single unit “that of washing to separate entrained solids in a dilute liquid stream and condensing components of the effluent stream, for example water, which in certain applications is beneficial to further processing in the recovery train.” The '136 published application does not however provide for any means of recovery and reuse of the catalyst particles that are concentrated in the bottom stream from the dual purpose quench zone described therein. This is particularly clear in the teachings of the sole example of the '136 published application which is contained in paragraph 0082.
The problem solved by the present invention is then in a nutshell to remedy the prejudices of the prior art and provide an OTO conversion process that utilizes a fluidized bed reactor system in combination with relatively expensive ELAPO-containing catalyst particles to enable not only the successful recovery of these product effluent-contaminating catalyst particles but also their reuse in the OTO catalytic conversion zone in order to recapture their catalytic activity. The invention thus substantially improve the economics of the overall process by diminishing the need for fresh catalyst make-up to compensate for this source of catalyst loss.